1. Field of the Invention
The present invention relates to a seismic exploration device whose configuration and/or features may be adapted or modelled according to particular conditions, and to a method for the implementing thereof.
2. Description of the Prior Art
Seismic exploration devices used for seismic prospecting generally comprise a reception device consisting of a very large number of seismic receivers which are placed in various locations of the explored zone for receiving the seismic waves reflected or refracted by the subsoil discontinuities, in response to vibrations transmitted into the ground by a seismic source. The receivers generally consist each of a combination of several elementary sensors (geophones, hydrophones) interconnected electrically and arranged in the explored zone so as to filter for example surface noises within the scope of seismic reflection prospecting runs. Each of these receivers provides a seismic xe2x80x9ctracexe2x80x9d which is an electric average of the signals produced by all the elementary sensors. The receivers may be distributed in the field or along a well, or arranged along a seismic streamer towed through the water by a ship. In modern reception devices, the groups of sensors are associated with signal acquisition boxes. Each box collects the signals delivered by one or several groups of sensors, to digitize, store and average several traces recorded successively. By order of a central control and recording station, each acquisition box transmits sequentially the stored traces to a recording device through one or several common communication channels (radio cables or channels). The lay-out of the various sensors of a single xe2x80x9cseismic tracexe2x80x9d has a considerable effect on the rejection of the surface noises. The sensors may be arranged at regular or unequal intervals in some cases to filter surface noises better without degrading useful signals. In onshore seismic prospecting, it is well-known to carry out xe2x80x9cnoise shootingsxe2x80x9d whose analysis allows the best relative arrangement of the sensors of a single trace to be defined.
These sensors may be associated with means for modifying the characteristics of the resultant signals, passive networks or local amplification modules to weight the sensors respective sensitivities or gains, active low-pass filters to attenuate undesirable frequency bands, etc. Various active or passive weighting processes are for example described in U.S. Pat. Nos. 2,698,927, 2,747,172, 3,400,783, 3,863,200, 3,863,201, etc.
An analog processor for modelling the frequency gain or spectrum of the various sensors allows a substantial attenuation of the noise level before the digitization of the useful signals in the acquisition boxes or in the central recording laboratory, and therefore make it possible to reserve the total dynamic range of the digitization circuit for processing these signals.
The major drawback of all analog pre-processing equipments lies in their rigid structure. The filtering parameters must be known in advance and, assuming that the reception device that is effectively set in the field allows these parameters to be changed, appropriate adaptations have to be carried out on the location of these local processors, which delays the recording operations. Moreover, the possible corrections which may be achieved before a xe2x80x9cshootingxe2x80x9d cannot be changed during recording.
Now, there are numerous cases, which will be shown in the description below, where trace adjustments and re-arrangements would be very useful adapting to the subsoil being explored and to obtain more readable seismic profiles. By extending the possibilities of existing recording laboratories, a solution could be considered to model a seismic reception device more easily. For example, if a 20- to 24-bit dynamic laboratory, capable of acquiring 500 to 4000 different channels (instead of the 12 to 15 bits currently obtained for the acquisition of 50 to 400 traces) was available, the analog filtering could be eliminated since it would be possible to acquire the signals of each of the elementary sensors distributed in the field with sufficient dynamic range. The optimum filtering of noise would be obtained from the recorded signals. However, the cost of such a laboratory with high dynamic range and a large number of traces, together with the cost of the processing of the large volume of data being obtained would be prohibitive.
The seismic exploration method according to the invention comprises installating, in a zone to be explored, an emission-reception device consisting of an array of receivers producing a set of seismic traces, a source of seismic signals and of at least one control and recording station. The invention is characterized in that each trace of at least part of the set of seismic traces is obtained through the combination of several seismic signals coming from several elementary sensors associated with control elements, and this combination may be modified at any time from the control and recording station by changing the combined elementary signals and/or by modifying the configuration of the elementary sensors going into the combination.
The method may comprise applying remote controls allowing for example:
differentiated control of the amplitude of the various signals constituting at least one of the traces, or
application of selective phase shifts to the signals received by the sensors constituting at least one of the traces, or
selection of the number of signals picked up constituting at least one of the traces, or
a selected combination of signals received by the sensors of at least one of said traces, such as convolutional or recursive filterings for example, to be obtained, or
inclusion of particular sensors in several adjacent seismic traces to be obtained.
The previous adjustments may be combined and also modified in time during the recording of a shooting for example.
Implementation of the method according to the invention allows the reception device to be configured at any time before each triggering of the seismic source or even during the reception of the seismic signals after the triggering of the source. The previous configuration, which are usual with reception systems of fixed configuration, are thus avoided or shortened. Various modifications or processings may be applied to the traces in real time during the recording sessions in the field without requiring a laboratory with extended processing capacity.
The seismic exploration device according to the invention comprises an array of seismic receivers producing a set of seismic traces, a source of seismic signals, and at least one control and recording station for collecting the seismic traces coming from the various receivers. It is characterized in that each receiver of at least one group of the array of receivers comprises a plurality of elementary seismic sensors associated with remotely controllable control elements, and means for combining the seismic signals resulting from the control elements, a remote control for remotely controlling the control elements so as to modify at will and at any time the configuration of the elementary sensors contituting each of the receivers and/or for modifying at least part of the elementary signals combined to produce each seismic trace coming from the group.
The device may comprise a assembly of acquisition apparatus distributed in the zone to be explored to collect each the signals of at least one seismic receiver, with at least one communication channel between the acquisition apparatus and the control and recording station for the transmission of control data and signals, and the remote control being connected to the remotely controllable control elements by means of the at least one communication channel and of conducting means arranged between them and the acquisition apparatus.
The control elements may for example comprise opto-electronic elements.
The conducting means reach for example the remotely controllable elements associated with arrays of sensors, the remote control means comprise means for coding to associate address words with control signals and the remotely controllable elements may also comprise address decoders. The conducting means comprise for example adjusting means.
The connections going from several sensors may also be split to associate each one with several adjacent traces.